rust on raspberries

I'd be very surprised if any fungicide would persist long enough to cause any problems in these circumstances, but I'm sure you would be equally surprised if I didn't advise you to always follow the instructions and warnings on all gardening chemicals.

More importantly, I'm not at all sure that any fungicides available to the home gardener will control it; you will have to do some more research and in the end maybe just hope that the improved air circulation after pruning will reduce the extent of the damage.